Due to the large variety of possible flaws (e.g. air inclusions, delaminations, gluing errors, impurities, kissing bonds) different methods than the established measurement methods of metal bonds have to be used for the inspection of composite laminates. Depending on the type of discontinuity the requirements of the inspection task are very diverse and time consuming. Due to the differences in the lateral and axial dimensions of defects and the attenuation varieties it is necessary to measure as broad-banded as possible. Typical band-widths as implemented in contact testing methods cannot be used in air-coupled ultrasonic testing at the moment.
We describe an efficient implementation of such an inspection with significantly improved results compared to conventional systems. It is based on a new multi-element air ultrasonic transducer. Each of the elements has different frequencies. The overlapping sound beams of the individual elements allow generating a broad-band signal which can be additionally steered inside the material as it is well-known from phased-array UT.
The new transducer increases the bandwidth considerably by an in-phase simultaneous activation of all elements. In order to operate such an ultrasonic transducer a special sender-receiver electronic is necessary. As the individual transducer elements have to be activated simultaneously the system must have at least as many channels as the ultrasonic transducer. Additionally all channels with different frequencies have to be activated with a very high time accuracy relative to each other.
The evaluation takes place under consideration of the dispersive properties of the inspected test objects with respect to the interdependency with ultrasound. Frequency-dependent properties like attenuation as well as geometry characteristics such as scattering behavior at interfaces cause a change in the measured ultrasonic signal. Accordingly, it is possible to achieve new contrast mechanisms. An increase of the effective bandwidth by combining a multi-element air-coupled ultrasonic transducer and a multi-channel measurement system allow gaining more information of the sample in only one measurement cycle. This accelerates the measurement of different frequencies drastically and eliminates uncertainties during the alignment of transducers with different frequencies.